A Study of the Vascular Actions of 5‐hydroxytryptamine, Tryptamine, Adrenaline and Noradrenaline

5‐Hydroxytryptamine (HT) is a more powerful constrictor of the pulmonary vessels of the dog and the cat and of the vessels of the rabbit's ear than adrenaline or noradrenaline. 2. Adrenaline and noradrenaline are more powerful constrictors of the hind leg vessels of the dog and the cat and of the renal vessels of the cat than HT. 3. HT is usually between 2 to 50 times as active as tryptamine. 4. Lysergic acid diethyl amide (L.S.D.) is a strong antagonist of the constrictor action of HT in both the lung and the leg vessels and has only a weak effect on the actions of adrenaline and noradrenaline. Ergotamine and tolazoline are relatively ineffective in modifying the vascular actions of HT. 5. Since HT is a substrate of amine oxidase, we expected to find that an inhibitor of this enzyme such as paredrine would potentiate the action of HT. This was not observed. However, tryptamine was found to potentiate the action of noradrenaline. 6. Desensitization to HT and to tryptamine by themselves was constantly observed.